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LIGHT EMISSION / DETECTIONLIGHT EMISSION / DETECTIONLasers and LED Passive ElementsLasers and LED Passive Elements

Piotr TurowiczPiotr TurowiczPoznan Supercomputing and Networking CenterPoznan Supercomputing and Networking Center

piotrekpiotrek@@man.poznan.plman.poznan.pl

Training SessionTraining Session

9-10 October 20069-10 October 2006.

http://www.porta-http://www.porta-optica.orgoptica.org

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LIGHT EMISSION / DETECTION

Transmissionchannel

Tx EO RxO

E

ReceiverConverterTransmitter Converter

The principle of an optical communication system

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Wavelength [nm]

Frequency [Hz]2x1014 3x1014 5x1014 1x1015

Infraredrange

Visiblerange

Ultravioletrange

Fiber optic transmission range

Glass

Plastic850 -1630 nm

520-850 nm

1800 1600 1400 1200 1000 800 600 400 200

Wavelength range of optical transmission

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Conversion from electricity to light is achieved by a electronic : LED (light emitting diode) VCSEL (Vertical Cavity Surface Emitting Laser) LASERS FP (Fabry - Perot)

That: changes modulated electrical signal in light modulated signal inject light into fiber media

From electricity to light

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Main characteristics for transmission purposes:

1 Central wavelength

(850/1300/1550)

2 Spectrum width (at ½ power)

3 Power

4 Modulation frequency

(consequence of slope)

1 Wavelength nm

Power dB

3

2Power/2

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Light emitters characteristics

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Spectrum of a LASER or LED source

+5 to -10dBm

LASER

1-5nm

λ

LED

Density-15 to -25 dBm

60-100nm

λ

Different frequency = different wavelength = different colors

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Is the level of light intensity available for transmission Average power is the mean value of the power during modulation

Power available for transmission is also function of:• Fiber core size• Numerical aperture Light entrance cone

N.A.(Numerical Aperture)

Power

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Modulated frequency

Is the rate at which transmission changes intensity (logical 0 to 1) Rate is function of time Time is function of slope Slope is characteristic of emitter (technology)

LED functions at lower frequency (longer time)LASERS at higher (shorter time)TIME influences modal bandwidth

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Emitters comparison

Type Cost Wavelength

(nm)

Spectral

width (nm)

Modulated

frequency

Power

(dBm)

Usage

LED $ 850-900

1250-1350

30-60

< 150

< 200 MHz - 10 to -30 F.O.

systems

Short

Wavelength

Lasers

$$ 780 4 ≥ 1GHz +1 to -5 CD

Fiber Ch.

VCSEL $$ 850

1300

1 to 6 ≈ 5GHz +1 to -3 F.O.

Giga speed

Lasers $$$ 1300

1550

1 to 6 ≥ 5GHz +1 to -3 F.O. SM

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Emitter characteristics transmission related effects

LED

VCSEL

LASER

Over Filled Launch (OFL)

Restricted Mode Launch (RMF)

Restricted Mode Launch (RMF)

• Emitters inject light into fiber under different conditions (emitter physical characteristic). Modes travel consequently Power is distributed consequently

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Emitters consideration

Splice

Multimode Fiber

MMSM

Activecomponent

Rec

TX

Rec

TX

Cabling

Generally, emitters can be optimized for fiber they have to illuminate for example to reduce effects of DMD - “Differential Mode Delay” .

1000Base-LX is used on MM as well as SM VCSEL cannot be optimized.

DMD optimization is achieved by Conditioned Patch Cords

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The principle of an optical communication system

Transmissionchannel

Tx EO RxO

E

ReceiverConverterTransmitter Converter

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From light to electricity

• Conversion from light to electricity is achieved by photodetector/receiver that: is triggered by modulated light transforms modulated light into modulated electrical signal

• Transmission characteristics are: Sensitivity Dynamic range BER

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Receiver characteristic

Sensitivity is the minimum power that is detected by the receiver with BER

level

BER is the max allowed error counted in bit in error/bit transmitted

BER is function of sensitivity among others characteristics

Dynamic range Is the maximum average power received to maintain BER

Too much power causes distortion and saturationToo less power causes no bit received

Both causes BER in excess of specified limit

Dynamic range is expressed as difference between min. and max.

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Spectral sensitivity of detectors

Material used in electronic manufacturing determine the sensitivityTechnology and temperature regulate response in amplitude and time (slope)

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Bandwidth limitations dependent on electronics

Switching time (or rise time, or slope) is affecting the width of the signal

Width of signal is determining the spreading of the signal Signal spreading is the cause of bandwidth limitations Bandwidth limitation in a fiber channel is therefore function of:

1. fiber bandwidth (known factor)

2. contribution of electronic (active components dependent)

3. Length of the channel (known or to be calculated)

Complex equationStandard

17http://www.porta-http://www.porta-optica.orgoptica.org

Passive FO elementsPassive FO elements

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Passive elements

Passive elements in Optical Network:

Optical fiber

Spliter/combiner

MUX/DMUX

Add MUX

Fiber Bragg grating based devices

Circulator

Isolator

Lens

Attenuator

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In general, multiplexer/demultiplexers for DWDM arerequired to have the following optical characteristics:

• Small center wavelength offset from grid wavelength

The permissible center wavelength offset depends on the transmission spectrum of the MUX and the transmission bit rate of the system, but is normally not more than 0.05 nm.

• Low insertion loss

As in the case of other FO transmission devices,insertion loss should ideally be as low as possible

• Low channel crosstalk

Channel crosstalk in terms of a specific MUX channel n is expressed as the difference between the insertion loss at the grid wavelength n of channel n and the insertion loss at the grid wavelength of the respective channel. Channel crosstalk should be as low as possible (-25 dB or better)

REQUIRED OPTICAL CHARACTERISTICS

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splittercombiner

1

2

3

4

coupler star coupler

λ1+λ2

λ 2

λ 1

wavelenght multiplekser

λ1+λ2λ 1

λ 2

wavelenght demultiplekser

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lustropółprzepuszczalne

F1

F2 F3

1

2

3

4

Fibers optic

mixer-rod

mixer-rod mirror

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Polishing coupler

Melting and stretching coupler

Coupler based on planar lightwave circuit (PLC) technology

Stage of coupler manufacturing

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MUX / DMUX

Fiber IN

Fiber OUTLens

Diffractiongratting

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Wavelenght filters

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1, 2

lens GRINFilter

b)

filter

1, 2

2

1

Fiber IN

GRIN lens

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Optical waveguide circuit structure of AWG

MUX

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Lens GRIN

Microptic elements:

GRIN Lens - GRadient INdex Lens

SELFOC - self focusing

NA2NA1

FiberSource

Dimensions:

Lenght: 3–30 mm

Diameter: 1-2 mm

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Isolator

Magnetooptic material

H - magnetic field strength

paramagnetic

lensmirror

lens

Optical prism

Optical prism

magnet

light beam

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Attenuators

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Fiber Bragg dispersion compensatorPrinciple

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References

Reichle & De-Massari